The use of chlorine dioxide produced by the reaction of sodium chlorate with a reducing agent in strong acid solutions to bleach pulp is an established technology with many variants. However, it has the drawback of requiring an external generating step which requires a costly generator. There has been considerable interest in the past in avoiding the generating step and obtaining the bleaching action of sodium chlorate by mixing the salt directly with the pulp slurry.
Addition of sodium chlorate in small amounts to chlorine water was suggested by Heirman, TAPPI 39 (11) 754-757 (November 1956), as a way of improving pulp strength and brightness. However, Marpillero, TAPPI 41 (5) 213A (1958), has stated that chlorate was quite ineffective when mixed with wood chips or a pulp, even at high concentrations. Conversely, several other investigators, viz., Kachalov etal., USSR Patent 112,344, published Jun. 25, 1958; Marpillero, TAPPI 41 (5), 213A (1958); Marpillero, "A.T I P. Bull ," (2):57-67 (1959); Robert et al , "Celluloza Hirtie," 10 (7/8):265-7 (1961) Societe Francais du Xylon, French Patent 1,478,302, published Mar. 20, 1967; Partridge, Hooker Chem. Corp., U.S. Pat. No. 3,770,646, issued Nov. 6, 1973; Zakoschikob et al., "Sb Jr. Mosk Jekhnol. Int.," (28):165-70 (1976); Chugunkin et al , . . "Sb. Jr. Mosk. Jekhnol Int, " (28): 176-80 (1976); Osipova et al., "Sb Jr. VNII Tsellyul-Bumazh Prom., " (58):25-38 (1971); Osipova, . "Sb Jr. VNII Tsellyul-Bumazh Prom., " (60):23-26 (1972); Osipova et Patent 368,365, published January 26, 1973; Osipova et al., . "Sb. Jr. VNII Gidvoliza Matev." (24) :109-118 (1974 ); and Janci et al , "Vyskum Prace Odburu Papiera, Celulozy," (20):V40-44 (1975), have claimed that the use of chlorate for pulping wood chips and bleaching pulp was possible by employing (1) an acid medium, or (2) a reducing agent ( i. e., tartaric acid, citric acid, or formaldehyde) plus a catalytic salt containing vanadium, titanium, molybdenum, arsenic, silver, manganese or chromium.
Marpillero, supra, also used mixtures of sodium chlorate, hydrochloric acid and vanadium to bleach pulps. Rapson et al., TAPPI 42(8) (1959) compared the use of this mixture of chemicals with the direct use of chlorine dioxide to bleach pulp and determined that it was more economical and less damaging to the pulp to first produce chlorine dioxide in a generator and then add it to the pulp.
A summary of the conditions for, and the results of, bleaching with "activated" chlorate (sodium chlorate containing one of the salts of the transition metals described above) is shown in Table I, below. Robert et al., supra, has observed that kraft pulp suffered serious degradation by bleaching with activated chlorate.
Chloric acid has been used directly for bleaching by Polcin et al., Czechoslavakian Patent 164634, published September 1976. They showed that, with vanadium as a catalyst and the addition of a mild reducing agent, chloric acid could bleach pulp, probably through the formation of chlorine dioxide. Indeed, when used in extreme concentrations at 90.degree. C for 6 hours, chloric acid was found to bring thin sprucewood chips to 87% brightness in 51% yield. However, there is little information in this publication on the effect of chloric acid on the carbohydrates in the pulp and the scope of the reaction.
Chloric acid is both a strong acid and a powerful oxidant. It is also an intermediate in the production of chlorine dioxide which, in turn, is used as a bleaching agent in the pulp and paper industry. Chloric acid is formed by acidifying a sodium chlorate solution with strong acid, e.g., sulfuric acid. The process generates sodium sulfate as a by-product, which may or may not be useable in other parts of the pulp and paper process.
Another method of generating chloric acid is by the action of a suitable acid, e.g., sulfuric acid, on a soluble chloric acid salt, e.g., barium chlorate, which forms a precipitable salt (barium sulfate) therewith. This method is of laboratory interest only.
There is considerable literature describing the preparation of chlorine dioxide from sodium chlorate. All this literature, however, describes the same initial step wherein sodium chlorate is treated with a strong acid before various reducing agents are applied to the resulting solution.
It is now possible to generate chloric acid using an electrolytic system (Lipsztajn et al., U.S. Pat. No. 4,915,927, issued Apr. 10, 1990), or using a membrane system involving stacked pairs of membranes by electro-dialytic ion separation of sodium chlorate (Paleologou et al., Proceedings Pacific Paper Expo., Vancouver, B.C., Canada (December 1990). With these new technologies, it is technologically feasible to use externally-generated chloric acid in accordance with this invention.